(1) Field of the Invention
This invention generally relates to RF antennas and more specifically to a class of antennas known as bicone antennas.
(2) Description of the Prior Art
RF bicone antennas are well known in the art. For example, U.S. Letters Pat. No. 2,175,252 (1937) to Carter discloses a bicone antenna in which a high frequency apparatus drives two conical antenna elements. The high frequency apparatus drives the conical elements at their proximate apices. In this specific antenna, four insulating rods are equiangularly spaced about the periphery of the conical sections at the edges thereof to position the two cones.
U.S. Letters Pat. No. 2,218,741 (1939) to Buschbeck discloses another bicone antenna for operating over broad band. In this antenna a set of parallel resistances connect between a common point and a plurality of spared positions around the base of each cone. The parallel resistances have an aggregate value which is equal to the surge impedance of the antenna.
In U.S. Letters Pat. No. 4,074,268 (1978) to Olson, an antenna has the structure of a conic monopole above a ground plane. A plurality of vertical modulator fins constitute controllable shorting elements. The antenna also includes a plurality of suppressor posts that are used to shape the field radiating from the antenna.
U.S. Letters Pat. No. 5,134,420 (1992) to Rosen et al. discloses a bicone antenna with an orthomode tee as an input/output terminal, an internal dielectric polarizer, a circular waveguide with eight longitudinal radiating slots, two 30.degree. conical reflectors, an external meanderline polarizer and a partial circular waveguide short. The partial circular waveguide short leaks a predetermined amount of radiation out of the end of the waveguide to fill the center hole of a doughnut-shaped radiation pattern to produce a hemispherical RF beam.
In U.S. Letters Pat. No. 5,367,312 (1994) to Masters, a biconical antenna is used to measure the intensity of incident RF electrical fields. This antenna comprises a pair of aligned rods with wires that define conical cavities around each of the rods. A ferrite choke surrounds each of the rods within the conical cavities to choke off resonances within the cavity.
Each of the foregoing references describes a different variation of a bicone antenna in the context of an antenna operating in isolation. That is, the antennas are not disclosed as being in proximity to other antennas. In a number of applications, however, antennas are often stacked, as on the mast of a ship. In such configurations, an antenna feed cable for one antenna must be routed past all antennas located below that one antenna. As will be evident, such cable routings are critical, because a misplaced cable can alter the radiation pattern of any antenna it passes.
For example, an IFF monocone now being used is a monocone fed monopole fed above a truncated ground plane in a stack of antennas. To allow a cable to pass this antenna, a quarter-wave shorted transmission line choke is placed on the axis of the monopole at its feedpoint to allow the cable to pass through the feed point of the antenna. The outer conductor of the transmission line connects to the monopole at its feedpoint; the center conductor of the transmission line, to the ground plane. The center conductor contains the cable to pass through the antenna. To allow the cable past the end of the monopole, the monopole is made hollow so that the inside of the monopole forms another .lambda./4 shorted transmission line. The first transmission line going up the monopole axis forms the center conductor and the inside of the monopole forms the outer conductor. Both are shorted near the feed point. Consequently an infinite impedance exists between the end of the monopole and the feed through cable at a quarter wavelength.
This approach imposes certain restrictions. For example, the chokes that mount on the antenna axis have a finite high impedance bandwidth. This bandwidth limits the choking action and the bandwidth where the impedance of the chokes does not change the matched impedance of the antenna thereby changing the bandwidth of the antenna.
Placing a quarter wave shorted transmission line choke on the antenna axis also requires feeding the antenna off axis. Consequently the feedpoint of the monopole becomes the lower end of the outer conductor of the axial choke transmission line. This produces asymmetrical azimuth patterns at higher frequencies.
It has also been found that the impedance bandwidth characteristics for such a monocone are reduced significantly. It becomes questionable as to whether the antenna with the chokes can be broadband matched. In one specific antenna, for example, the bandwidth for a VSWR of 2:1 is about 11%. For a VSWR of 3:1 it is about 28%. Both are considerably less than the bandwidth characteristics for a monopole without such chokes. With bicone antennas, it is possible to obtain infinite bandwidths above a certain "cut-in" frequency, i.e., the frequency at and above which the VSWR becomes a low, flat VSWR about an antenna Z.sub.o value.
The use of shorting elements in antennas is also known. For example, bifilar and quadrifilar helix antennas often contain a wire or plate of low inductive impedance that is close to being a short circuit across the end of the helix. These shorts rotate the reflection coefficient by approximately 180.degree. without any significant change in the VSWR (relative to the antenna Z.sub.o) above the antenna cut-in frequency. Moreover, patterns emanating from the antenna remain approximately the same although there may be some increase in back side radiation. The primary purpose of using such elements is to allow a coaxial feed cable to be introduced onto the antenna at a zero RF point and to then use the antenna as an infinite balun to allow the cable to be brought to and connected to the antenna's feed point. In other antennas, a cylindrical slot can be shorted at both of its ends at least a quarter of a wavelength from its center where likewise a feed cable can be introduced onto the antenna structure at one of the shorts. An endfire slot normally extends on the non-radiating side of its feed point for less than a quarter of a wavelength and is then shorted so that a coaxial feed cable can also be introduced onto the antenna to feed the antenna.
Similar elements are used in both dipole or slot antennas. An early submarine antenna, known as an AS-1288/BPXIFF antenna, comprises a monocone above a ground plane. This antenna includes nearly ideal short circuits at its ends between the top and the ground plane to physically support the cone. Partial radial short circuits exist above these shorts and probably provide a small amount of isolation between the shorts to ground and the monocone. Likewise, the part of the monocone below the radial short circuit tapers from the inside diameter of the radial short, which can provide some isolation to ground. The small opening of this antenna at the shorts and the close to zero impedance of the shorts probably raise the cut-in frequency of the unshorted antenna appreciably and make the antenna have a limited impedance bandwidth.